1. Field of the Invention
This invention relates to a low-contamination method for comminuting solid silicon fragments. In particular, the invention relates to a method in which a heating and cooling step is utilized to produce stress fractures in a silicon block prior to comminuting.
2. Description of the Prior Art
During the production of silicon wafers, which are the material for electronic components or solar cells, various substeps require the silicon to be used in a relatively finely divided form and often also in certain grain size ranges. This applies, in particular, to those process steps in which the silicon present in solid form is melted down in melting crucibles. The melted silicon is then reconverted to the solid state from the molten state with the development of the particular desired crystallized structural features. An example of this is the Czochralski crucible pulling technique in which a seed crystal is used to pull a monocrystalline silicon ingot of a particular crystallographic orientation from a silicon melt. The ingot is then eventually sawn into silicon wafers. Other examples are casting processes in which molten silicon is poured into molds or crystallization chambers and caused to solidify. In these casting processes, a polycrystalline product is obtained which is made up of monocrystalline regions oriented in a particular manner and, in doing so, develops a structure frequently described in technical language as a columnar structure (see, for example, U.S. Pat. Nos. 4,382,838 and 4,312,700). Various methods of pulling silicon ribbons or foils, for example, the method taught in U.S. Pat. No. 4,447,289, should also be noted.
An essential reason why the silicon is not used in coarse-fragment form in these processes, before melting, is that the volume of the melting crucible cannot be satisfactorily utilized. This is because with such a coarse material, the space is not filled efficiently. Even if the crucible is initially completely filled with coarse fragments of silicon, when melted, the crucible is generally only filled to about 30 to 50% with melted-down material. In addition, the reduction in volume of the silicon, in the order of about 10% which occurs during the melting process, must also be added to this reduction. At the same time, the entire crucible has to be heated so that only a comparatively small quantity of melt is obtained from the use of a large volume and a high energy input.
These disadvantages have been overcome by using finely divided to granular silicon which makes possible an appreciably more efficient utilization of the crucible volume provided. Such material may, for example, be produced by further comminuting ingots of pure polycrystalline silicon obtained by gas-phase deposition. These ingots typically have a length of 1 to 2 m and a thickness of 10 to 30 cm and are comminuted with the aid of steel jaw crushers or crushing mills after they have first been split into coarse fragments weighing typically about 0.05 to about 10 kg. The material obtained as a result of this operation, which varies within a wide grain size spectrum, is then, as a rule, sorted by metallic sieves, for example, into various grain size fractions. The intensive contact of the hard, brittle silicon particles with metallic materials, which is unavoidable in this operation, inevitably results in a contamination by abrasion. This contamination manifests itself all the more intensely, the finer the grain size of the silicon particles produced. Therefore, for applications in which the highest purity is of importance, such as in the Czochralski crucible pulling technique or in casting solar-cell base material, only silicon above a particular minimum grain size is generally used to keep the risk of entrainment of contaminants as low as possible. An appreciable proportion of the comminuted material, which is prepared at considerable expense, is therefore not available, or available only to a limited extent, for its originally intended application.
U.S. Pat. No. 4,565,913, assigned to Komatsu Electronic Metals Co., Ltd., discloses a known method for comminuting ingottype and billet-type polycrystalline semiconductor silicon in which the contamination problems are avoided. This method teaches briefly heating the ingots dielectrically from the interior with microwaves and shattering them. The fragments are collected in pure water, pure water optionally being sprayed on the periphery or the outer edge of the heated polycrystal. The particle size of the comminuted silicon can, in this case, be determined only by the size of the ingot-type polycrystal and the order of magnitude of the microwaves and cannot, therefore, be readily influenced. In U.S. Pat. No. 4,565,913, yet a further method is mentioned in which the ingots are heated to a high temperature by an external heating furnace and then thrown into water so that they are broken by the shock of rapid cooling. A disadvantage of this method is the unavoidable contamination of the broken parts through contact with other materials. In addition, there is only a slight possibility of influencing the size in which the fragments are finally produced on shattering.